1. Field of the Invention
The present invention relates to a dual-wavelength laser apparatus for generating a laser beam consisting of two wavelength components.
2. Description of the Prior Art
Since a laser beam of a specific wavelength tends to be absorbed in a certain gas, the presence or absence of the gas can be detected therewith, as is well known to those skilled in the art. Sensing techniques using this principle have been in widespread use in a variety of applications such as industrial measurements and pollution monitoring. A good example is a He-Ne laser which has two component lines of 3.3922 .mu.m (.lambda..sub.1) and 3.3912 .mu.m (.lambda..sub.2). The wavelength .lambda..sub.1 is strongly absorbed by methane, but the wavelength .lambda..sub.2 is little absorbed thereby. The differential absorption at these two wavelengths provides a sensitive method of detecting the presence of methane. Since methane is the major constituent of city gas, methane gas leak detection allows city gas leak detection.
U.S. Pat. No. 4,489,239 describes a methane leak sensor of this type. FIG. 1 of this prior art patent illustrates a sensor system using two He-Ne lasers of 3.3922 .mu.m and 3.3912 .mu.m. In this sensor system, two mechanical choppers modulate laser beams of different wavelengths from the two He-Ne lasers at different frequencies. The modulated laser beams propagate in the air, and the components in outputs from a sensor synchronized with the modulation frequencies are detected by a lock-in amplifier. The presence or absence of methane is then detected according to an output ratio.
According to another conventional methane leak detection technique, a single chopper wheel or a chopper-mirror wheel is used to alternately emit two different wavelength components. In this case, if these wavelength components have equal intensity, the frequency component synchronized with the chopper detected by the lock-in amplifier gives the difference of absorbance between the two wavelengths and allows methane leak detection.
The methane leak detection system of this type uses a large number of mirrors and beam splitters making its optical system complex and bulky. In addition, optical alignment is cumbersome and the laser beam loss is large. Deviations in optical misalignment due to vibrations, temperature changes, or the like must be taken care. Signal processing is complex and high-frequency modulation cannot be achieved owing to the limitation in operation of the mechanical chopper, thus decreasing the signal to noise ratio. In the former example, two choppers and two lock-in amplifiers are required (in addition to the two lasers), thereby increasing the overall size of the apparatus. In the latter example, it is difficult to tune the two different wavelength components to equal amplitudes, because the outputs are unbalanced by, for example, temperature changes.
Still another conventional sensor system which solves some problems of the sensor system with two He-Ne lasers is proposed in FIG. 4 of the prior art U.S. patent described above. In this conventional sensor system, a He-Ne plasma tube is placed in an invar-stabilized cavity composed of three mirrors. Two of those mirrors constitute a Fabry-Perot interferometer which selects the wavelength. The piezoelectric disc oscillates one of the mirrors, thereby achieving alternate intensity modulation at the two wavelengths.
Since a chopper is no longer necessary with dual-wavelength laser, the overall structure can be simplified. In this case, a cell containing a small amount of methane may be inserted into the cavity in order to equalize the gain at both laser wavelengths so that equal power is generated at each wavelength. However, since no feedback mechanism is provided, the outputs of the two wavelength components cannot be completely equalized and may be unbalanced owing to, for example, temperature changes.
As cited in lines 56-62, column 9 of the prior art patent described above, the paper "Improved Use of Gratings in Tunable Lasers," by J. E. Bjorkholm, T. C. Damen, and J. Shaw (published in Optics Communication, Vol. 4 (1971), p. 283 (1971)), describes a three-mirror cavity. In this cavity, a half mirror is arranged just in front of a grating to improve the reflectivity of the grating and to increase the output. In the arrangement, the half mirror and the grating constitute a Fabry-Perot interferometer. The mirror position must be controlled very precisely to achieve a high reflectivity.